This invention relates to a torsion bar which is used for a slow-action rotation shaft device which supports a second member which rotates relatively with respect to a first member.
Such torsion bar for a slow-acting rotation shaft device is used for a switch device at a rotatably supporting portion of a compact case, toilet, radio cassette, a dash board of a car or the like.
Such slow-acting rotation shaft device includes a shaft 3 which rotatably supports a rotative member 1 which relatively rotates with respect to a fixed member 2 as shown in FIG. 4.
The shaft 3 is, as shown FIG. 5 and FIG. 6, includes a hollow shaft body 31, cap members 32 and 33 outwardly inserted to both side portion of the shaft body 31 rotatably, two torsion bars 4 and 4 latched one end of hook portion 41 into the shaft body 31 and another end of hook portion 42 into the cap member 33, and a viscous grease (not shown) enclosed between shaft body 31 and cap members 32 and 33.
The shaft body 31 is provided with a stepped shaft having small diameter shaft portions 31b, 31b outwardly inserted the cap members 32 and 33 rotatably to both sides of a large diameter shaft portion 31a of the middle portion thereof and further provided with a slit penetrated hole 34 in a length direction. A key 35 is projectingly formed at the large diameter porion 31a. Each of cap members 32 and 33 is formed as a cylinder having a bottom wherein the inner diameter thereof is the same as the outer diameter of the small diameter shaft portion 31, while the outer diameter thereof is the same as the outer diameter of the large diameter shaft portion 31a, and the key 37 and 38 are projectingly formed at the outer circumference of each cap member 32 and 33.
At the bottom of the cap member 33, a concave slit 36 is formed. Each of cap members 32 and 33 is outwardly inserted into each of small diameter shaft portions 31b and 31b and the ring grooves 32a and 33a are inserted into ring rib 31c of each small diameter shaft portion 31b to intend to stop the slipping out, whereby the cap members 32 and 33 are combined with shaft body 31 rotatably.
In this combined state, torsion bars 4 and 4 are inserted into the penetrated hole 34 of the shaft body 31, one end hook portion 41 and another end one 42 are latched to the penetrated hole 34 and the concave slit 36 of the cap member 33 respectively to combine therewith, and the viscous grease is enclosed between cap members 32 and 33 and each small diameter portion 31b.
In shaft 3 thus combined, the large diameter shaft portion 31a is positioned in the supporting portion at bearing portions 11 and 12 of the rotatative member 1 respectively, and each key 35, 37 and 38 is inserted into the key grooves (not shown) formed at rotatably supporting portion 21 and bearing portions 11 and 12 respectively, thereby attaching the large diameter portion 31a to a rotatably supporting portion 21 and cap members 32 and 33 to the bearing portions 11 and 12 respectively in an unrotatable state.
In the conventional slow-acting rotation shaft device thus constructed, though the rotative member 1 rotates by the torque of the torsion bars 4, 4, a viscous resistance of the viscous grease occurs at the time of the rotation, the rotation member 1 can be rotated with a slow speed in spite of the torque load of the torsion bars 4,4.
In a thus slow-acting rotation shaft device, since it is necessary to rotate the rotative member 1 against the viscous resistance of the viscous grease, the torsion bar 4 used must be one which can provide a torque necessary for the above rotation. In order to obtain such torque, the diameter of the wire rod of the torsion bar could be increased, but, this increases the stress of the torsion bar itself which causes a permanent set or a breakage. Accordingly, as described above, heretofore a plurality of torsion bars 4 having a small wire rod diameter and the same length have been used to obtain the torque described above.
However, since this conventional torsion bar for a slow-acting rotation shaft device uses a plurality of torsion bars 4 having the same length, when the torsion bars 4 are twisted, ends of hook portions 41, 41 and the other ends of hook portions 42, 42 of the torsion bars interfere with each other and it is impossible to obtain good operation.
Thus, this invention is needed. The object is to provide a torsion bar for a slow-acting rotation shaft device without using a plurality of torsion bars.